This invention relates to structural laminates which comprise two metal sheets or two metal skin layers with a polymer core disposed between and attached to each of the inside surfaces of the metal sheets. The laminates of this invention have improved structural strength compared to prior laminates of this type. The laminates of this invention are also useful in decorative and protective applications as well as structural applications.
In the field of laminates, metal-resin-metal laminates are disclosed in U.S. Pat. No. 4,313,996 to Newman, et al. and U.S. Pat. No. 4,601,941 to Lutz, et al. Additional laminates are disclosed in U.S. Pat. No. 3,382,136 to Bugel, et al., U.S. Pat. No. 3,392,045 to Holub, U.S. Pat. No. 3,455,775 to Pohl, et al., U.S. Pat. No. 3,594,249 to Mueller-Tamm, et al., U.S. Pat. No. 3,623,943 to Altenpohl, et al., U.S. Pat. No. 3,655,504 to Mueller-Tamm, et al., U.S. Pat. No. 3,952,136 to Yoshikawa, et al., U.S. Pat. No. 4,330,587 to Woodbrey, U.S. Pat. No. 4,369,222 to Hedrick et al., U.S. Pat. No. 4,416,949 to Gabellieri, et al., U.S. Pat. No. 4,424,254 to Hedrick et al., U.S. Pat. No. 4,477,513 to Koga, and U.S. Pat. No. 4,594,292 to Nagai, et al. In these references, a property which is generally important is that the laminates be formable, particularly thermoformable. Other properties which have been important for metal-resin-metal laminates have been the resistance of the metal skin to heat, weather, chemicals and impact, as well as the metal skin's hardness, impermeability and strength. Multi-layer laminates have been made with multiple, alternating layers of resin and metal. Laminates in this field have been used also for heat insulation and vibration damping.
In U.S. Pat. No. 3,499,819 to Lewis, the resin core is a polypropylene which contains a foaming agent additive to cause the polypropylene to form a foam between the metal layers. In U.S. Pat. No. 3,560,285 to Schroter, et al. a mixture of polyether polyols is reacted with polyisocyanate and a blowing agent to form foamed urethane cores between metal layers. U.S. Pat. No. 4,421,827 to Phillips discloses metal-clad articles which use a combination of thermosetting resins and particular adhesives to bond a resin layer to a metal facing.
As disclosed by Vogelesang (Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 492-496) aluminum laminates with high tensile strength and fatigue resistance have also been made with multiple core layers of aramid fiber-reinforced epoxy resins. These aluminum laminates, known as "ARALL" laminates, have been developed for use as aircraft skins. See also U.S. Pat. Nos. 4,489,123 and 4,500,589 to Schijve, et al.
While metal-resin-metal laminates have been used in trailer bodies, such laminates have not been used for structural members. Examples of such non-structural laminates are sandwich structures which have been used for thermal insulation in trailer bodies, as shown in U.S. Pat. Nos. 3,363,231 to Grosgebauer, et al., and 4,482,589 to Widman. In U.S. Pat. No. 2,730,722 to Jones, a metal-resin-plywood laminate is disclosed that is both insulative and structural, but is over 2 inches (5 cm) thick. Thermal insulation panels usually have very low density cores, i.e., specific gravities in the range of 0.10 or less, and thus a low shear modulus and low impact resistance.
For trucks operated on public highways, United States federal law now limits the exterior width of trailers to 102.38 inches (260 cm). Therefore, interior space can be increased if the thickness of the walls of the trailer body can be decreased. This is particularly important in industries transporting light, bulky loads, such as empty beverage cans, where the volume of the trailer is often more limiting than the weight of the load. Thus, emphasis in this field is on designing trailers for maximum interior usable width. Also, because of the dimensions of standard pallets, it is desirable that the interior width of the trailer be at least 101 inches (256.5 cm). The advantages of having thin walls and the additional interior space provided by thin-wall trailer construction are discussed by Pennington, Modern Metals, November, 1986, pp. 20-25.
In order to achieve the desired interior width dimension of at least 101 inches (256.5 cm), each sidewall of the trailer body, including rivet heads, can be no thicker than 0.69 inches (1.75 cm). Sheet and post trailers, the most common type, follow a truss design which generally uses about 0.045 inch (0.114 cm) or 0.050 inch (0.127 cm) aluminum sheet riveted to steel or aluminum frame and posts. The sheet and post trailer has an overall wall thickness of 1.4 inches (3.56 cm) to 1.7 inches (4.32 cm) and a maximum interior width of 99.58 inches (252.9 cm) to 99 inches (251.4 cm). Conventional trailer bodies made with resin-fiberglass-plywood laminates use either 0.625 inch (1.6 cm) or 0.75 inch (1.9 cm) plywood as the core, resulting in an overall thickness of up to 0.86 inches (2.18 cm) even in a post-free design using that type of panel.
In U.S. Pat. No. 4,212,405 to Schmidt there is disclosed a trailer having a "box" type of design using unitary aluminum alloy plates as the walls and supporting structure in a frameless trailer body. This structure is intended to provide an interior trailer dimension of at least 101.5 inches (257.8 cm). While this configuration provides the desired wall thickness of down to about 0.156 inch (0.4 cm), but usually about 0.25 inch (0.6 cm), a trailer made of the aluminum alloy plate is much heavier than desired. There are other disadvantages, as well, in having a trailer wall made of a unitary aluminum alloy plate. Since these plates are typically a single 9 ft. (2.74 m) by 44 ft. (13.41 m) rigid plate, there are associated problems in manufacturing, shipping and handling of the plates, as well as in assembling and repairing the trailer. It is particularly difficult to repair these trailers without losing interior space and without either decreasing wall thickness or, if the damage is repaired by welding, without loss of strength.
In order to overcome these disadvantages, Banerjea discloses an alternative trailer construction having a box type design in U.S. Pat. No. 4,685,721. This trailer body construction uses smaller sized individual plates spliced together with joining panels and stiffener panels designed to be riveted with the aluminum sheets to keep the wall thickness to a minimum. Since the additional splices add undesired weight, some of the panels or sheets are thinner in lower stress areas than in higher stress areas in order to partially compensate for the added weight from the splices and joining panels. The overall result, however, is a trailer that is still heavier and has more susceptibility to denting of the thinner side panels than desired, and the interior width of which is less than desired because of the splices and rivets.
The disclosures of the above references are incorporated herein by reference.